First Detection of Chromospheric Magnetic Field Changes During an X1-Flare

TL;DR

This study reports the first detection of stepwise magnetic field changes in the chromosphere during an X1 solar flare, revealing larger and more complex alterations than in the photosphere, with implications for understanding flare dynamics.

Contribution

It provides the first spectropolarimetric measurements of chromospheric magnetic field changes during a flare, highlighting differences from photospheric changes and challenging existing models.

Findings

Chromospheric B$_{ m LOS}$ changes are stronger and cover larger areas than photospheric ones.

Chromospheric changes often decrease in magnitude, unlike photospheric increases.

Changes are near, but not exactly co-spatial with HXR emission and sunquakes.

Abstract

Stepwise changes of the photospheric magnetic field, which often becomes more horizontal, have been observed during many flares. Previous interpretations include coronal loops that contract and it has been speculated that such jerks could be responsible for sunquakes. Here we report the detection of stepwise chromospheric line-of-sight magnetic field (B$_{\rm LOS}$) changes obtained through spectropolarimetry of Ca II 8542 \AA\ with DST/IBIS during the X1-flare SOL20140329T17:48. They are stronger ($<$640 Mx cm$^{-2}$) and appear in larger areas than their photospheric counterparts ($<$320 Mx cm$^{-2}$). The absolute value of B$_{\rm LOS}$ more often decreases than increases. Photospheric changes are predominantly located near a polarity inversion line, chromospheric changes near footpoints of loops. The locations of changes are near, but not exactly co-spatial to hard X-ray (HXR) emission and neither to enhanced continuum emission, nor a small sunquake. Enhanced chromospheric and coronal emission is observed in nearly all locations that exhibit changes of B$_{\rm LOS}$, but the emission also occurs in many locations without any B$_{\rm LOS}$ changes. Photospheric and chromospheric changes of B$_{\rm LOS}$ show differences in timing, sign, and size and seem independent of each other. A simple model of contracting loops yields changes of opposite sign to those observed. An explanation for this discrepancy could be increasing loop sizes or loops that untwist in a certain direction during the flare. It is yet unclear which processes are responsible for the observed changes, their timing, size and locations, especially considering the incoherence between photosphere and chromosphere.